In the past few years, the general public has become increasingly apprehensive of the impact man-made waste has on the environment. Hence there is a growing interest in developing novel biodegradable (and preferably compostable) plastics from renewable resources.
One particularly interesting candidate for this task is poly(hydroxy carboxylic acid), in particular poly(lactic acid) (PLA), now commercially available on a relatively large scale. The lactic acid is obtained from plants such as corn and sugar-cane or other sugar- or starch-producing plants. Not only is PLA obtainable from renewable materials, it is also industrially compostable. For these reasons, there is significant interest in using PLA as a substitute in applications, where petroleum-based thermoplastics have conventionally been used.
Unfortunately, PLA used on its own does not have the same advantageous properties as conventional plastics do. In particular PLA has performance problems related to heat resistance, brittleness and limited flexibility, resulting in poor mechanical strength. On the other hand, polyolefins, such as polyethylene, have much better mechanical properties. It has been attempted to combine these properties by blending PLA with polyethylene to obtain a resin that is at least partially obtainable from renewable resources, but still has acceptable mechanical properties. However, it is known that blending PLA with conventional polyethylenes such as Ziegler-Natta-catalysed polyethylenes provides heterogeneous resin blends, due to the differences in polarity and molecular weight distribution of the two components. In the past, compatibilising agents were used to increase the homogeneity of the blends. However, this requires an additional industrial step, as well as specific conditions during extrusion. Furthermore, the addition of compatibilising agents is expensive and changes the properties of the desired product. Thus both the compatibilising agent and the by-products change the properties of the desired end product, be it a film, fibre or moulded object.
EP 1 777 263 A also teaches mixing polyolefins with PLA by using a compatibiliser, wherein the compatibiliser is a hydrogenated, diene-based polymer containing at least one functional group selected from carboxyl group, acid anhydride group, epoxy group, (meth)acryl group, amino group, alkoxysilyl group, hydroxyl group, isocyanate group and oxazoline group. The polyolefin is a polymer obtained by polymerizing ethylene and/or at least one kind of alpha-olefin using either the high-pressure method or the low-pressure method. Examples of the alpha-olefin includes alpha-olefins of 3 to 12 carbon atoms, such as propylene, 1-butene, 1-pentene, 3-methyl-1-butene, 1-hexene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3-ethyl-1-pentene, 1-octene, 1-decene, 1-undecene and the like.
US 2005/0192405 A discloses a polymer alloy of PLA and polyolefins. The two components are made miscible by including a polyalkylacrylic ester and/or a polyvinyl ester, as well as a block copolymer of a polyalkylacrylic ester and a polyolefin and/or a block copolymer of a polyvinyl ester and a polyolefin. The polyolefin described is either a polyethylene obtained by a radical polymerisation mechanism or a polyethylene or polypropylene obtained via cationic addition polymerisation mechanisms using Ziegler-Natta catalysts.
It is hence an object of the invention to develop a polyethylene-based resin that is at least partially obtainable from renewable resources and has better or at least similar mechanical properties than hitherto known blends of polyethylene with resins obtainable from renewable resources.
It is also an object of the invention to develop a resin that is at least partially obtainable from renewable resources and has improved mechanical properties in comparison with poly(hydroxy carboxylic acid)s.
Additionally, it is an object of the invention to develop a resin that is at least partially obtainable from renewable resources and has similar mechanical properties to polyethylene.
It is further an object of the invention to develop a resin that has better gas barrier properties than polyethylene.
It is also an object of the invention to develop a resin with better surface tension properties than polyethylene.
Furthermore, it is an object of the invention to blend polyethylene with poly(hydroxy carboxylic acid)s without having to use compatibilising agents to obtain homogeneous blends.
It is also an object of the invention to find a resin blend at least partially composed of materials from renewable resources that can be used in film, thermoforming, blow moulding, injection stretch blow moulding, extrusion blow moulding and/or rotational moulding applications.
At least one of the above objects is achieved with the implementation of the current invention.